Vascular access presents one of the major problems affecting the success of hemodialysis treatment. Currently, vascular access failure is a major cause of morbidity for hemodialysis patients. Graft access thrombosis occurs in 60% of patients within one year and more than 30% of all grafts fail within 18 months after placement. Vascular access complications are the major cause of morbidity in the hemodialysis population, accounting for more than 15% of hospitalizations.
To maximize the longevity of a vascular access, Kidney Disease Outcomes Quality Initiative, i.e., K/DOQI, practice guidelines suggest an aggressive policy for monitoring vascular access patency for the purpose of extending the life of a monitoring access and minimizing thrombosis. Conventional devices diagnose the presence of pathology by measuring parameters, such as access flow and access recirculation. These conventional devices may employ a variety of techniques, for instance ultrasound dilution techniques. However, these devices and methods provide only indirect evidence of the access site and of the degree of access pathology.
One technique for monitoring a vascular access is referred to as ultrasound Doppler imaging. The ultrasound Doppler imaging technique provides an image of the access flow, which provides different information than the dilution method. However, a major problem of the ultrasound Doppler imaging technique is operator error. For instance, this technique requires that measurements be taken at the same location of a patient at different times. There is significant difficulty in identifying the same location of the patient when a subsequent measurement is taken. Computerized X-ray tomography (“X-ray CT”) is another technique for monitoring a vascular access to provide an image of an access area. However, both the ultrasound Doppler imaging technique and X-ray CT are expensive methods which cannot be routinely used in clinical practice. Further, these imaging techniques cannot continuously monitor a vascular access during hemodialysis.
Another technique for monitoring a vascular access is electrical impedance tomography. Electrical impedance tomography provides an image based on a distribution of conductivity in a cross sectional area. Since organs and fluid volume, such as blood, have very different conductivity, and since particularly vascular grafts have a relative lower conductivity than blood vessels, in the arm, vascular grafts can be easily identified by electrical impedance tomography. Moreover, a dynamic image of a vascular access can be obtained by electrical impedance tomography.
One advantage of electrical impedance tomography is the ability to continuously measure changes in blood flow in the regional area with much lower cost, easy operation and portability. For instance, relative to other techniques, e.g., X-ray CT and positron emission tomography, electrical impedance tomography is significantly less expensive to perform and smaller in size. In addition, electrical impedance tomography is non-invasive, and employs a very weak electrical current on the skin, e.g., 0.25-5 mA. Furthermore, because electrical impedance tomography may produce many, e.g., thousands, of images per second, electrical impedance tomography may be employed to measure and monitor a vascular access continuously.
One problem that is experienced with electrical impedance tomography is that, because electric current flow in the body does not progress in straight lines but rather tends to spread out in all directions, electrical impedance tomography provides low spatial resolution. In addition, electrical impedance tomography does not consist merely of information relating to a measurement plane, e.g., a plane of conductivity distribution at which a measurement is taken, but also includes significant contributions of information from outside the measurement plane.
One method that may be employed in order to improve the resolution of an image in a cross-sectional area, e.g., the cross-sectional area of a vascular access, is to increase in the number of electrodes in the same area. However, a large number of electrodes positioned on a limited circular surface, e.g., an inner circumference of a vascular access, will reduce the distance between adjacent electrodes. As a result, there may be significant errors in the measurements provided by the electrodes because of the effect of skin impedance on the measurements. This is especially problematic for the purpose of three dimensional imaging, which in conventional systems employs numerous electrodes in close proximity relative to each other. Errors of individual electrode location and the connecting interface between electrode and skin are major sources of measurement error that significantly reduce the resolution of an image generated by electrical impedance tomography.
Currently, there are two major models which are used to generate, e.g., reconstruct, an image. A first model that can be used to reconstruct an image is referred to as “the forward problem”. In the first model, there is provided a resistivity distribution with a boundary current and voltage, and there is calculated the internal current and voltage distribution. To provide a solution to the forward problem, the first model employs a Finite Element Method (“FEM”) algorithm that is used to reconstruct the image.
A second model that can be used to reconstruct an image is referred to as the ‘problem’. In the second model, there is provided the boundary current and voltage and an internal resistivity distribution is calculated. To provide a solution to the inverse problem, the second model employs a back projection algorithm to calculate the resistivity distribution.